Positive feed pneumatic power drilling machines are widely used in many industries. Such drilling machines normally have a single motor for turning a spindle through a drive gear train. The spindle is threaded into a feed gear that turns at a predetermined rate faster than the turning rate of the spindle for advancing the spindle as the drill progresses through a work piece. The feed gear is driven by a gear train from the same motor as the drive gear train. The gear ratio of the drive gear train is selected to be slightly less than the gear ratio of the feed gear train so the feed gear will turn slightly faster than the drive gear. In that way, the spindle is advanced a predetermined amount for each turn. Once the spindle has been advanced sufficiently, a mechanism is actuated to disengage the spindle feed gear train from the motor and lock it in place. As the motor continues to drive the spindle in the same direction, the spindle threads turn inside the locked feed gear to rapidly retract the spindle.
Prior art mechanisms for disengaging the gear trains from the motor have included mechanical switches for interrupting the supply of compressed air to the motor. The inclusion of mechanical switches in a pneumatic drilling machine have certain disadvantages. First, they are difficult to assemble, requiring delicate placement of the moving parts within the switch. Further, the delicate parts of the switch are prone to wear and tear, and detract from the longevity of the drilling machine as a whole, which otherwise benefits from a reduction in the number of moving parts that pneumatic tools generally provide.
Another aspect of prior art drilling machines is that they include pneumatic counting devices, for counting the number of drive cycles carried out by the machine. This allows the owner to carry out the required maintenance on the machine at a proper interval. However, a feature of the prior art counters is that they typically have been configured to add one cycle to the total count each time the motor is switched on. This is disadvantageous because a drill user will often turn the drill off, and then on again, a number of times in the middle of the feed mode. Thus, a single feed cycle may be counted as a number of cycles. This has the undesirable effect of indicating that the drill has been used more often than it really has been, and leads to uneconomical servicing of the machine.
Thus, a need exists in the art for a pneumatic drill with a pneumatically operated switch for turning off the motor. A need also exists for a counting system that counts feed cycles of the drill only at the completion of a feed cycle. It is believed that the present invention addresses these and other needs.